1. Field of the Invention
The present invention relates to a reflective liquid crystal display (LCD) device and more particularly, to an array substrate of the reflective liquid crystal display (LCD) device and a manufacturing method for the same.
2. Discussion of the Related Art
The demand for flat panel display devices, which have properties, such as a shallow depth, light weight and low power consumption increases, as the information age rapidly evolves. Flat panel display devices are classified into one of two types of devices depending on whether the devices emit light. One type is a light-emitting type that emits light to display images and the other type is a light-receiving type that controls transmission of an external light source to display images. Plasma display panels (PDPs), filed emission display (FED) devices and electro luminescence (EL) display devices are examples of the light-emitting type of flat panel display devices. Liquid crystal display (LCD) devices are an example of the light-receiving type of flat panel display devices. The liquid crystal display device is widely used in, for example, notebook computers and desktop monitors because of its superior resolution, color rendering capability and high contrast in displaying images.
Generally, a liquid crystal display device has upper and lower substrates, which are spaced apart and face each other. The upper substrate includes at one least electrode and the lower substrate includes a plurality of electrodes. The at least one electrode of the upper substrate faces the electrodes of the lower substrate. Liquid crystal is positioned between the facing electrodes of the upper and lower substrates. A voltage is applied across the liquid crystal via the facing electrodes of the substrates such that alignment of the liquid crystal molecules changes corresponding to the applied voltage to display images.
Because the liquid crystal display device cannot emit light as discussed above, an additional light source is needed to display images. Accordingly, the liquid crystal display device has a back light behind a liquid crystal panel that is used as a light source. An amount of light transmitted through the LCD devices from the back light is controlled according to the alignment of the liquid crystal molecules to display images. The electrodes of each substrate are formed of transparent conductive material and the substrates are formed of a transparent material, such as glass. LCD devices that control the amount of light transmitted from a back light are transmissive liquid crystal display devices. Because the transmissive liquid crystal display device uses an artificial light source, it can display a bright image even in dark surroundings. However, the transmissive liquid crystal display device has high power consumption because of the power needs for the back light.
A reflective liquid crystal display device has been suggested to overcome the power consumption problem of the transmissive liquid crystal display device. A reflective liquid crystal display device controls a transmittance of a light, such as ambient light or artificial light, which is received through the upper substrate and reflected by the lower substrate back through the upper substrate depending upon the alignment of liquid crystal molecules. Accordingly, the reflective liquid crystal display device uses less power than a transmissive liquid crystal device since externally available light or ambient light is used instead of a light source powered by the display device. Unlike the transmissive liquid crystal display device, the electrodes on the lower substrate of the reflective liquid crystal display device are formed of an opaque conductive material that has a high reflectance. The structure of a related art reflective liquid crystal display (LCD) device will be described hereinafter with reference to attached figures.
FIG. 1 is a cross-sectional view of a related art reflective liquid crystal display (LCD) device. As shown in FIG. 1, the reflective liquid crystal device has a lower substrate 11 and an upper substrate 21 that are spaced apart from each other. A gate electrode 12 is formed on the lower substrate 11. A gate insulating layer 13 is formed on the gate electrode 12 and across the surface of the lower substrate 11. A gate line (not shown) is also formed beneath the gate insulating layer 13. An active layer 14 is formed on the gate insulating layer 13 above the gate electrode 12. Ohmic contact layers 15a and 15b are formed on the sides of the active layer 14. A source electrode 16b and a drain electrode 16c are formed on the ohmic contact layers 15a and 15b, respectively. The gate electrode 12, the source electrode 16b and the drain electrode 16c are parts of a thin film transistor T. A data line 16a is formed of same material as that of the source and drain electrodes 16b and 16c on the gate insulating layer 13 and connected to the source electrode 16b. 
A pixel region is defined between data lines 16a and 16d and gate lines (not shown) that cross the data lines. A passivation layer 17 is formed over the gate insulating layer 13 and the thin film transistor T. A contact hole 17a formed in the passivation layer 17 exposes a portion of the drain electrode 16c. A pixel electrode 18, such as a conductive reflective electrode, is formed on the passivation layer 17 in the pixel region and connected to the drain electrode 16c through the contact hole 17a. The pixel electrode 18 is formed over the thin film transistor T. The pixel electrode 18 overlaps the data lines 16a and 16d to increase the aperture ratio of the reflective LCD device. The passivation layer 17 can be formed of organic material having a low dielectric constant to prevent signal interference between the pixel electrode 18 and the data lines 16a and 16d. 
FIG. 1 also shows a black matrix 22 formed beneath the upper substrate 21. A red color filter 23a, a green color filter 23b and a blue color filter 23c are repeatedly formed beneath the upper substrate 21 and adjacent to the black matrix 22. A common electrode 24 is formed beneath the color filters 23a, 23b and 23c. A transparent conductive material, such as Indium-Tin-Oxide (ITO), is used as a common electrode. Each of the color filters 23a, 23b and 23c corresponds to a pixel electrode 18 on the lower substrate 11. The black matrix 22 overlaps edges of the pixel electrode 18. Because the pixel electrode 18, which is formed of opaque conductive metal material, covers the thin film transistor T, the black matrix 22 does not have to be formed such that it covers the thin film transistor to prevent light from interfering with the active layer 14.
As shown in FIG. 1, a liquid crystal layer 30 is interposed between the pixel and common electrodes 18 and 24. If a voltage is applied across the pixel electrode 18 and common electrode 24, the electric field across the liquid crystal between the pixel electrode 18 and common electrode 24 changes the alignment of molecules in the liquid crystal. Alignment layers (not shown) are formed on the pixel electrode 18 and beneath the common electrode 24 to initially align the molecules of the liquid crystal.
Images are displayed in the reflective liquid crystal display (LCD) device by forming the pixel electrode of material that has a high reflectability such that incident light on the pixel electrode that travels through the upper substrate 21 and the liquid crystal 30 is reflected back through the liquid crystal 30 and the upper substrate 21. Accordingly, the reflective liquid crystal display (LCD) device can display images in bright light conditions with little power consumption. Because the reflective electrode of the reflective liquid crystal display (LCD) device usually has a flat surface, the reflective electrode has a mirror reflection in that an incidence angle and a reflection angle are the same. Accordingly, the luminance of the reflective liquid crystal display device will depend, for a given direction from the device, upon a position of the light source. Therefore, it has been suggested that a reflective liquid crystal display (LCD) device includes a scattering film to scatters the light into many directions such that luminance is not as dependent on the position of the light source.
FIG. 2 is a cross-sectional view of a related art reflective liquid crystal display (LCD) device having a scattering film. FIG. 2 has all of the elements of FIG. 1. Further, FIG. 2 also includes a front scattering film 40 that is formed on the top side of the upper substrate 21, which is opposite to the side of the upper substrate 21 on which the black matrix 22 is formed. However, in case of the front scattering film 40, image blurring can occur due to back scattering of the displayed image from the front scattering film. Thus, the resolution of the displayed is decreased.
FIG. 3 is a cross-sectional view of a related art reflective liquid crystal display (LCD) device having an uneven reflective electrode. FIG. 3 has all of the elements of FIG. 1. However, the pixel electrode 18 in FIG. 3 has an uneven shape to scatter the light. The pixel electrode 18 to is formed to have an uneven surface by depositing it on a passivation layer 17 having an upper portion that is unevenly formed. The unevenness of the pixel electrode 18 varies the angle of reflection across the surface of the pixel electrode such that luminance of the related art reflective liquid crystal display (LCD) device in FIG. 3 is not dependent upon the position of a light source.
FIG. 4 is a graph illustrating light-paths and angles for the related art reflective liquid crystal display (LCD) device having the uneven surface on the pixel electrode. FIG. 4 shows that incident light I from the external environment 55 refracts when it passes through the glass substrate of the upper substrate 54 and the liquid crystal layer 53 and then reflects at a surface of the pixel electrode 52 on the lower substrate 51. The refraction index of the external environment is 1.0. The refraction index for the upper substrate 54 and the liquid crystal layer 53 is considered to be 1.5. The incident light “I” from an external light source comes into the upper substrate 54 with an incidence angle of α (alpha) and refracts as a first internal light II with a refraction angle of β (beta) due to the differences in refraction index between the external environment 55 and the upper substrate 54. Because the refraction indexes of the upper substrate 54 and the liquid crystal layer 53 are same, the first internal light II does not refract at the interface of the liquid crystal layer 53 and the upper substrate 54. The first internal light II comes into the uneven surface of the pixel electrode 52 with an incidence angle of γ (gamma) and then reflects as second internal light III from the uneven surface of the pixel electrode 52 with a reflection angle of γ. The incidence angle of the first internal light II and the reflection angle of the second internal light III are measured with respect to a normal line that is perpendicular to a tangent line of the uneven surface of the pixel electrode 52. The second internal light III goes out through the liquid crystal layer 53 and the upper substrate 54 as out-going light IV without refraction as shown in the FIG. 4 in this case.
The out-going light IV in FIG. 4 should be perpendicular to the surface of the second substrate 54 to increase the luminance of the display in a direction perpendicular to the surface of the upper substrate 54 of the reflective liquid crystal display (LCD) device. Further, the second internal light III also needs to be perpendicular to the upper substrate 54 such that back diffraction does not occur at the interface of the external environment 55 and the upper substrate 54. Because the light source is usually disposed such that incident light I has an incident angle of approximately 30° (degrees) with respect to a vertical direction of the upper substrate 54, usually the refraction angle as calculated in accordance with the Snell's law, is about 20° (degrees). Accordingly, the reflection angles γ should be about 10° (degree) so that the reflected second internal light III is perpendicular to the upper substrate 54. An inclination angle θ (theta) of the uneven surface of the pixel electrode 52 should be about 10° (degree) when the reflection angle γ is about 10° (degrees). The inclination angle θ is an angle that is measured between the tangent line of the uneven surface of the pixel electrode 52 and the horizontal direction of the lower substrate 51. Accordingly, it is desirable to form the pixel electrode to have a surface with an inclination angle of approximately 10° (degrees).
FIGS. 5A to 5B are cross-sectional views illustrating a fabricating sequence for curved profiles of an organic insulating layer in a reflective liquid crystal display (LCD) device according to a related art. In FIG. 5A, a plurality of organic film patterns 62 is formed at predetermined intervals by coating organic material on a substrate 61 and then patterning it. Varying the size of the organic film patterns and the interval between the organic films patterns controls an inclination angle of the curved profile that will later be formed. The organic film can be formed of a photosensitive material. Depending on whether the photosensitive material is negative or positive type, a portion of the photosensitive material that is exposed to light or a portion of the photosensitive material that is not exposed to light is removed. The organic film patterns 62 can be formed by coating additional organic material on the passivation layer 17 of FIG. 3 such that the passivation layer is used as a substrate for forming curved profiles. In the alternative, organic film patterns can be formed by patterning an upper portion of the passivation layer 17 of FIG. 3.
Referring to FIG. 5B, an insulating layer 63 that has the curved profiles 63a is formed by heating the organic film patterns 62 shown in FIG. 5A. More particularly, the organic film patterns 62 of FIG. 5A are melted by the heat and then cured to have an inclination angle θ (theta) of approximately 10° (degree). When a conductive material, such as metal, is deposited and then patterned on the insulating layer 63 having the curved profiles 63a to form a pixel electrode having the contour of the curved profiles is formed. However, it is hard to form the curved profiles 63a repeatedly to have the inclination angle θ (theta) of 10° (degree) according to the above process and accordingly the reproducibility is not good.
FIGS. 6A to 6C are cross-sectional views illustrating a fabricating sequence for curved profiles of an organic insulating layer in a reflective liquid crystal display (LCD) device according to another related art. As shown in FIG. 6A, a plurality of organic film patterns 72 is formed on a substrate 71, such as a passivation layer, by coating organic material on the substrate 71 and patterning it. In the alternative, the organic film pattern 72 can be formed by patterning an upper portion of the passivation layer. As shown in FIG. 6B, a first insulating layer 73 having curved profiles 73a is formed by heating the organic film patterns 72 and then curing it. As shown in FIG. 6C, a second insulating layer 74 is formed by coating an organic material on the curved profiles 73a. The inclination angle θ (theta) of the curved profile of the second insulating layer 74 can be controlled to be about 10° (degree) by controlling the curved profile 73a of the first insulating layer 73. Each curved profile of the second insulating layer 74 has a hemispheric shape that is symmetric with respect to a centerline as shown in the FIG. 6C. Because an inclination angle θ (theta) of the curved profiles 73a of the first insulating layer 73 does not need to be about 10° (degree) in this case, the reproducibility of the curved profiles 73a is relatively high compared to the process of FIGS. 5A to 5B.
FIG. 7 is a graph illustrating an effective reflection area of the curved profile of the pixel electrode for the reflective liquid crystal display (LCD) device according to the related art. As stated before, the curved profile of the pixel electrode forms a hemispheric curve. If the light source is positioned at a left side of the reflective liquid crystal display (LCD) device, the light comes in from the left side. If one curved profile of the reflective electrode is considered, the light comes in from a left side of the curved profile of the reflective electrode as shown in FIG. 7. Accordingly, the effective reflection area that can reflect the incident light toward the front of the reflective liquid crystal display is a surface of the curved profile of the reflective electrode on the left side of the curved profile with respect to the dotted line of FIG. 7. Thus, reflection efficiency is lower because a small area on one side of the curved profile is used.
In the case of the above process in FIGS. 6A to 6C, it is possible to reproduce the curved profiles but the process is complex due to the need to evenly or consistently deposit the second organic layer 74. Furthermore, the additional formation of the second insulating layer 74 on the first insulating layer 73 increases a total thickness of the insulating layer. Moreover, as in the case of FIGS. 5A to 5B, an interval between two neighboring organic film patterns 72 and a size of each organic film pattern 72 has to be carefully controlled to obtain a desired curved profile 73a for the first insulating layer 73. The consideration of the melting properties of the first insulating layer 73 together with the interval and the size of the organic film pattern 72 can cause reproducibility problems in terms of forming substantially equivalent curved profiles.